Traditionally when both the linear and nonlinear signal of the same frequency is required in a transducer, two separate transducers are utilized. An applicable transducer device uses two separate transducers to produce the linear and nonlinear signals. However, since the parametric pump frequencies utilized are quite high, the high frequency array is small and is located directly in front of the low frequency array. A problem in the design is the difficulty in making the high frequency transducer very small in order to be acoustically transparent to the linear transducer array.
An alternate system using a similar arrangement attempts to get around this problem by separating the high and low frequency transducer with a pressure-release sheet. In theory the sheet is rigid at low frequency operation so that the high frequency transducer vibrates in unison with the low frequency transducer. At high frequency operation the sheet decouples the transducers so that only the high frequency transducers vibrates. A drawback to this system is the difficulty in obtaining a suitable pressure-release sheet.
Another approach to the problem has been developed and utilizes an impedance matching stub on the face of the radiator to generate a second resonance. It has been standard practice in the past to add a quarter wave stub of an appropriate material on the face of a transducer to broaden the mechanical Q of the transducer. What this design has done is exploit the resonance of this stub to produce a higher frequency transmitting band. The disadvantage of this method is that the separation of the two resonances is generally limited to 1 to 2 octaves and as the separation increases, the bandwidth about the resonances decreases.